The present invention relates to a length- or angle-measuring arrangement.
Such absolute or incremental position-measuring systems may be used, for example, in processing machines which measure the relative position of a tool with respect to a workpiece to be processed or in coordinate-measuring machines which determine position and/or dimensions of test objects.
German unexamined patent specification OS No. 17 73 403 discloses that measuring errors will result in a length-measuring arrangement having a graduation carrier whose graduation plane containing the graduation lies outside the neutral plane of the graduation carrier when the graduation carrier bends while in its use position.
In one such length-measuring arrangement the graduation carrier for the graduation has, for example, a rectangular cross section, whose one surface forms the graduation plane with the graduation. Since this graduation plane with the graduation lies outside the neutral plane on the graduation carrier, bending of the graduation carrier in its use position creates expansions or compressions of the graduation plane with the graduation causing measuring errors. The neutral plane of the graduation carrier is the sole plane which does not undergo any change in length, either by expansion or compression upon bending of the graduation carrier.
German Pat. No. 25 10 219 describes a lengthmeasuring arrangement with a graduation carrier whose graduation plane with the graduation lies outside the neutral plane of the graduation carrier. The graduation is scanned by a scanning unit which is coupled to an object to be measured and to an auxiliary guide independent of the guidance of the object to be measured. To avoid measuring errors resulting from the bending of the graduation carrier in its use position, the scanning unit is coupled to the object to be measured at the height of the neutral plane of the graduation carrier articulately by means of coupling elements. Such an articulated coupling of the scanning unit in the neutral plane of the graduation carrier is relatively expensive and space consuming, and therefore may not be used in all measuring arrangements.
In order to avoid such measuring errors arising from the bending of the graduation carrier, there have been proposed graduation carriers with a U-shaped cross section or with an H-shaped cross section, wherein the neutral plane forms the graduation plane with the graduation.
Such an arrangement of the graduation plane with the graduation in the neutral plane of the graduation carrier nevertheless, however, yields measuring errors from the bending of the graduation carrier in measuring systems in which the graduation of the graduation carrier is scanned by a scanning unit in the direct light process with relatively great scanning distance and in which, therefore, the graduation plane forms with the graduation of the graduation carrier a plane of reflection. The parallel light beams produced in a scanning unit from a light source by means of a condenser pass through the scanning graduation on the scanning plane, fall upon the graduation plane with the graduation of the graduation carrier, and are reflected from this graduation plane as a reflection plane back onto the scanning graduation plane with the scanning graduation. The graduation plane of the graduation carrier acts, accordingly, as an optical element that images the scanning graduation again onto the scanning plane. Through the counter-running movement of the graduation of the graduation carrier and of the image of the scanning graduation on the graduation plane of the graduation carrier, the light beams are modulated and transformed by way of a photo-element into an electrical periodic scanning signal. The signal is fed to an evaluating means which generates measurement values representing the relative positions of the two objects.
The graduation plane of the graduation carrier and the scanning graduation plane of the scanning unit face one another at a certain parallel spacing, a. On a bending of the graduation carrier, there is produced in an arbitrary place of the graduation plane, an incline of angle .alpha., which displaces the image of the scanning graduation by an interval 2.alpha.a on the scanning graduation plane in one direction. If the graduation carrier with the graduation is shifted in the opposite direction through the interval .alpha.a, the displacement of the image of the scanning graduation through the interval 2.alpha.a on the scanning graduation plane is nullified. Thus, the inclination angle .alpha. causes a measuring error .alpha.a.
An object of the present invention is therefore to eliminate measuring errors in a length- or angle-measuring arrangement in which a graduation lying in a graduation plane of a graduation carrier lies outside the neutral plane of the graduation carrier. Other objects of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings.